1. Field of the Invention
The present invention relates to a working fluid composition for refrigerating machine, and more specifically to a working fluid composition for a compression-type refrigerating machine used in electric refrigerators, air conditioners, etc.
2. Discussion of the Related Art
Recently, the use of dichlorodifluoromethane (CFC12) for refrigerators and car air conditioners is to be legally banned in order to protect the ozone layer, and also the use of chlorodifluoromethane (HCFC22) for room air conditioners is about to be legally regulated. Thus, hydrofluorocarbons which do not destroy the ozone layer, such as difluoromethane (HFC32), have been developed as substitutes for HCFC22.
However, since the polarity of hydrofluorocarbons is higher than that of CFC12, the use of conventional lubricating oils, such as naphthenic mineral oils, poly-.alpha.-olefins, or alkylbenzenes, causes two-layer separation of the working fluid at low temperatures. This is due to poor compatibility between the conventional lubricating oils and hydrofluorocarbons. Two-layer separation hampers oil return, which in turn interferes with heat transfer due to deposition of a thick oil film around the condenser and evaporator used as heat exchangers. It can also cause important failures such as poor lubrication, and foaming upon starting operation. Therefore, the conventional refrigeration oils cannot be used as refrigeration oils under these new refrigerant atmospheres.
As for lubricity, CFC12 generates hydrogen chloride upon its partial decomposition. The hydrogen chloride thus formed reacts with the friction surface to form a coating of chlorides, thereby the lubricity being improved. On the other hand, hydrofluorocarbons containing no chlorine atoms are not expected to have such an effect; therefore, the refrigeration oils used in combination with hydrofluorocarbons are required to have a further excellent lubricity when compared to the conventional refrigeration oils.
In addition, the refrigeration oils used in combination with hydrofluorocarbons have to have good thermal stability in the presence of hydrofluorocarbons.
Moreover, in the compression-type refrigerating machines for electric refrigerators, since organic materials are used for motor components such as insulators and enameled wires, the working fluid comprising a hydrofluorocarbon and a refrigeration oil is required to have no adverse effects to these organic materials and also have a good insulating property.
Polyether compounds which can be used as refrigeration oils in combination with hydrofluorocarbons such as 1,1,1,2-tetrafluoroethane (HFC134a) are disclosed in U.S. Pat. No. 4,755,316, WO 90/05172, European Patent No. 377,122, and other publications. In addition, for instance, mixed oils of polyether oils with ester oils which can be used as refrigeration oils in combination with 1,1,1,2-tetrafluoroethane (HFC134a) are disclosed in U.S. Pat. No. 4,851,144 (corresponding to Japanese Patent Laid-Open No. 2-276894) and Japanese Patent Laid-Open No. 2-158693; ester oils are disclosed in G.B. Patent No. 2,216,541 and European Patent No. 406,479; and carbonate oils are disclosed in European Patent Nos. 421,298 and 452,816 and Japanese Patent Laid-Open No. 3-217495.
Since these compounds have a higher polarity than the naphthenic mineral oils, their compatibility with HFC134a at low temperatures is certainly good. Also, these compounds have good thermal stability in the presence of the hydrofluorocarbons. However, when compared with the CFC12-mineral oil system which is a conventional working fluid, both the hydrofluorocarbons and the oils have high polarity, so that they are likely to absorb water. Thus, these compounds lead to poor thermal stability in the presence of HFC134a and cause organic materials such as PET films to be hydrolyzed. Particularly in the case of using an ester oil, a carboxylic acid is likely to be formed, and the formed carboxylic acid may in turn corrode metals to wear down the metals. Also, in the case of using a carbonate oil, there arises such a problem that a non-condensable carbon dioxide gas is generated owing to hydrolysis of the carbonate oil.
Therefore, a method of adding an orthoester or acetal compound to a polyether oil or ester oil as a moisture-trapping agent has been proposed (see Japanese Patent Laid-Open No. 4-320498). However, the acetal compound derived from a monohydric or dihydric alcohol disclosed in this publication has an undesirably low molecular weight, low boiling point, and low flash point. Moreover, from the viewpoints of providing a viscosity necessary for a lubricating oil and having compatibility with hydrofluorocarbons, the amount to be added is restricted.